This invention relates to an improvement in the manufacture of color cathode ray tubes. Particularly this invention is useful in manufacturing color cathode ray tubes having a so-called black matrix screen which is provided with a light-absorbing film forming a "black surround" filling the gaps between the color dots on the inner surface.
In the color cathode ray tube with the above-mentioned black matrix screen, the room light incident upon the screen surface is absorbed by the light-absorbing film, and accordingly, any decrease of contrast in the television picture due to the room light incident upon the screen surface is eliminated. In the color picture tube of this type, it is usual to provide firstly a light-absorbing film having a number of apertures on the inner surface of the face panel, and afterwards, a color-emitting phosphor dot is formed in each aperture.
There are two types of the above-mentioned color cathode ray tube; namely, a first type tubes wherein color phosphor dots are formed to have larger diameters than those of the electron beams which pass through the apertures of the mask and strike the dots, and a second type tube wherein color phosphor dots are formed to have smaller diameters than those of the electron beams which pass through the apertures of the mask and strike the dots. In manufacturing the second type tube, first a preparatory shadow mask is produced, which is initially provided with apertures smaller in diameter than those to be provided in the finished state of the shadow mask. By using this preparatory shadow maks, an ultraviolet ray exposure process is performed to form the light-absorbing film, i.e., a black matrix. By the subsequent process, the apertures of the shadow mask are enlarged by a secondary etching to a specified size as required in the finished tube.
According to a novel manufacturing method which was recently developed by applicant, entitled "direct exposure method", the diameters of the apertures of the light-absorbing film, and hence the diameters of the color dots formed in the apertures, can be made smaller than those of the apertures of the shadow mask without the above-mentioned secondary etching, by using a specially converged light-beam for exposure and by controlling the condition of development of the exposed dots. In accordance with this method, the subsequent enlarging of the apertures by secondary etching can be omitted. The present invention is useful when combined with such direct exposure method.
A typical known and conventionally used manufacturing process for the production of a color cathode ray tube is described in greater detail with reference to FIG. 1 which is a cross-sectional view showing the parts of a color screen exposure apparatus, and to FIG. 2 which is a graph showing the distribution of light-transmission (penetration) of a filter 5 used in the apparatus of FIG. 1.
In FIG. 1, a photosensitive slurry, made by blending a 1 to 7% polyvinyl alcohol aqueous solution with ammonium dichromate at a weight ratio of between 1:0.005 and 1:0.2, is evenly applied on the inner wall of a face panel 2 and is dried to form a photosensitive film 1, namely, a film of which the parts exposed to a specified amount of ultraviolet rays are hardened so as to be retained, i.e., developed, while the remaining parts are washed away. Then, a shadow mask 3 is installed at a predetermined position on the face panel 2. Subsequently, the photosensitive film 1 is exposed for a predetermined time to ultra-violet rays, which are derived from a point-light source 4 situated at a specified off-axis position in relation to the face panel, which position is called a deflection center of the tube. The rays are then passed through a light-attenuator 5, path-refracting lens 6 and the apertures of the shadow mask 3. Exposures in the above-mentioned way are made in three stages, namely, for red, green and blue dots, respectively, by placing the point-light source 4 at each deflection center for red, green and blue electron beams, respectively. Thus, a plurality of exposed points numbering three times the apertures of the shadow mask 3 are produced on the photosensitive film 1. Next, the shadow mask 3 is removed from the face panel 2, and the photosensitive film 1 is washed and developed in a hot or cold water shower. Therefore, a number of polyvinyl alcohol dot films are formed on the inner surface of the face panel 2. These dots are hereinafter called PVA dots, as generally known.
Next, a slurry of light-absorbing substance, such as aquadag or a substance which changes into a light-absorbing substance by heating, is applied to the inner surface of the face panel 2, and is dried. Then, the face panel 2 is immersed in a hydrogen peroxide bath, so that the above-mentioned PVA dots are dissolved and removed, simultaneously removing the light-absorbing substance remaining on the top of the dots. Thus, a light-absorbing film 1 with a number of apertures is formed on the inner surface of the face plate 2.
Color phosphor dots are to be applied in the apertures of the light-absorbing film 1, and accordingly, the areas of the color phosphor dots are defined by the diameters of the PVA dots. The diameters of the PVA dots are highly dependent on the extent of exposure, since the photosensitively of the polyvinyl alcohol-ammonium dichromate slurry is very high and the light-transmission into the slurry is also very high.
On the other hand, a good color balance of the picture on the screen is dependent on the uniformity of the sizes of the three phosphor dots of primary colors in each small area. Accordingly, in order to obtain good color balance across the picture screen, a uniformity of exposure is required throughout the screen.
In the conventional manufacturing method, a light-attenuator 5 having light-transmission distribution as shown in FIG. 2 was provided between the path-refracting lens 6 and the point-light source 4, as shown in FIG. 1.
In order to attain the required uniformity of exposure, the transmission rate at the edge parts F of the light-attenuator 5 was selected to be higher than that of the central part b, as shown in FIG. 2, and preferably, the light-attenuator 5 was rotated around the axis of the face panel 2 in order to avoid unevenness around the axis. One example of such prior art method was disclosed in the specification of the U.S. Pat. No. 3,259,038 of G. A. Burdick, et al., patented on July 5, 1966.
However, even with the use of such a light-attenuator, attainment of desired uniformity of exposure was not adequate since the light-attenuator 5 was situated beneath the path-refracting lens 6 and accordingly was situated too far from the shadow mask 3. If an edge part d on the photosensitive film is exposed to the light beam c from the light source 4 so as to form the PVA dots corresponding to the green phosphor dots and next is exposed to the light beam e from the light source 4 then situated at a position 4', as indicated by dotted line in FIG. 3, so as to form the PVA dots corresponding to red or blue phosphor dots, then the former light beam c passes through a point A of the light-attenuator 5 and the latter light beam c passes through another point B of the light-attenuator 5. As is shown in FIG. 2, light-transmissions of the point A and of the point B differ significantly. Accordingly, the exposure by the light beam c and that by the light beam e differ prominently from each other, so that attainment of exposure uniformity becomes impossible. This variation naturally causes considerable variation in the size of the phosphor dots in the small area d and hence causes deviation of the color from the proper color at the edge parts of the picture screen.